Glossary

The quality of place being pleasant or attractive; agreeableness. A feature that increases attractiveness or value, especially of a piece of real estate or a geographic location

Aquifer

A sub-surface zone or formation of rock or soil containing a body of groundwater.

Attenuation

Reduction of peak flow and increased duration of a flow event.

Attenuation storage

Volume used to store runoff during extreme rainfall events attenuating flows by limiting flow rates out of it. Comes into use once the inflow is greater than the controlled outflow.

Balancing pond

A pond designed to attenuate flows by storing surface water runoff during the storm and releasing it at a controlled rate during and after the storm. The pond always contains water.

Baseflow

The sustained flow in a channel or drainage system not directly caused by rainfall.

Basin

A ground depression acting as a flow control or water treatment structure that is normally dry and has a proper outfall, but is designed to detain stormwater temporarily.

Best Management Practice (BMPs)

A range of measures designed to reduce the rate and quantity of surface water runoff from developed areas and to improve runoff water quality. Stormwater BMPs are techniques, measures or structural controls used to manage the quantity and improve the quality of stormwater runoff. Term used in USA although often with less emphasis on amenity and biodiversity.

Biodegradable

Capable of being decomposed by bacteria or other living organisms.

Biodegradation

Decomposition of organic matter by micro-organisms and other living things.

Biodiversity

The diversity of plant and animal life in a particular habitat.

Bioretention area

A depressed landscaping area that is allowed to collect runoff so it percolates through the soil below the area into an underdrain, thereby promoting pollutant removal.

Block paving

Pre-cast concrete or clay brick sized flexible modular paving system.

Brownfield site

A site that has been previously developed (see previously developed site).

California bearing ratio (CBR) value

An empirical measure of the stiffness and strength of soils, ratio (CBR) value used in road pavement design.

Catchment

The area contributing surface water flow to a point on a drainage or river system. Can be divided into sub-catchments.

Catchpit

A small chamber incorporating a sediment collection sump which the runoff flows through.

Climate change

Climate change refers to any significant change in measures of climate (such as temperature, precipitation, or wind) lasting for an extended period (decades or longer).

Combined Sewer Overflows

Overflow systems built into combined sewer networks which allow a certain amount of flow to discharge directly into a watercourse untreated, to ensure the sewer network does not become surcharged in storm conditions.

Combined sewer

A sewer designed to carry foul sewage and surface runoff in the same pipe.

Consent

Permission (usually conditional) granted by the environmental regulator; usually associated with the discharge of flow, or potentially polluting flow, to a watercourse or into the ground.

Construction (Design and Management) Regulations 2007 (CDM)

Construction (Design and Management) Regulations 2007, which emphasise the importance of addressing construction health and safety issues at the design phase of a construction project.

Construction Quality Assurance (CQA)

A documented management system designed to provide adequate confidence that items or services meet contractual requirements and will perform adequately in service. CQA usually includes inspection and testing of installed components and recording the results.

Contaminated ground

Ground that has the presence of such substances that, when present in sufficient quantities or concentrations, are likely to have detrimental effects on potential targets.

Contributing area

The area that contributes storm runoff or other output to the receiving system.

Control structure

Structure to control the volume or rate of flow of water through or over it.

Controlled waters

Waters defined and protected under the Water Resources Act 1991. Any relevant territorial waters that extend seaward for 3 miles from the baselines, any coastal waters that extend inland from those baselines to the limit of the highest tide or the freshwater limit of any river or watercourse, any enclosed dock that adjoins coastal waters, inland freshwaters, including rivers, watercourses, and ponds and lakes with discharges and groundwaters (waters contained in underground strata).For the full definition refer to the Water Resources Act 1991.

Conventional drainage

The traditional method of draining surface water using subsurface pipes and storage tanks.

Conveyance

Movement of water from one location to another.

Conveyance capacity

The capacity of a system to convey flow. In piped systems, the conveyance capacity will exceed the pipe full capacity due to flow backing up in manholes.

Critical duration event

The duration of rainfall event likely to cause the highest peak flows at a particular location, for a specified return period event.

Culvert

A closed conduit carrying a watercourse beneath an obstruction such as road, railway or canal.

Curtilage

An area of land around a building or group of buildings which is for the private use of the occupants of the buildings.

De-watering

The removal of groundwater/surface water to lower the water table.

Degradation

The process of breaking down materials (pollutants) to a less complex/lower state.

Deposition

Laying down of matter via a natural process.

Design Statement

In the context of a planning application, a written statement to a local authority prepared by an applicant setting out the design principles adopted in relation to a proposed design for a site and its wider context.

Design and Access Statement

Formal documents explaining the design philosophy which is behind a planning application.

Design criteria

A set of standards agreed by the developer, planners, and regulators that the proposed development should satisfy. See drainage design criteria.

Design storm

A synthetic rainfall event of a specific profile, intensity and period of time for a given duration and return period; derived by statistics and recorded rainfall events for a specific location.

Designing for Exceedance

An approach that aims to manage exceedance flows during rainfall events, eg the use of car parks during extreme events.

Detention basin

A vegetated depression that is normally dry except following storm events. Constructed to store water temporarily to attenuate flows. May allow infiltration of water to the ground.

Detention pond/tank

A pond or tank that has a lower outflow than inflow. Often used to prevent flooding.

Diffuse pollution

Pollution arising from land-use activities (urban and rural) that are dispersed across a catchment, or sub-catchment, and do not arise as a process effluent, municipal sewage effluent, or an effluent discharge from farm buildings.

Discharge

The flow rate of liquid passing through a conduit.

Discharge consent

Permission to discharge surface water or effluent, subject to conditions laid down by an environmental regulator.

Drainage design criteria

A set of performance levels agreed by the developer, planners, and regulators that the proposed drainage system should satisfy.

Drainage stage

A component of a sustainable drainage system.

Dry swale

Shallow vegetated channel with filter in the base to convey surface runoff to the sewer network or infiltrate into the surrounding soils.

Duration

The time period over which an event occurs or has an impact.

Ecology

All living things, such as trees, flowering plants, insects, birds and mammals, and their habitats.

Ecosystem

A biological community of interacting organisms, and their physical environment.

Ecosystem services

Services provided by the natural environment that benefit people.

Environmental regulators

These include the Environment Agency in England and Wales, the Scottish Environment Protection Agency in Scotland and the Northern Ireland Environment Agency.

Erosion

The natural processes of weathering, dissolution, abrasion, corrosion, and transportation, by which solid material is worn away at a location and deposited elsewhere, usually by wind or water.

Eutrophication

Water pollution caused by excessive plant nutrients that result in reduced oxygen levels. The nutrients are powerful stimulants to algal growth which in turn use up oxygen in water. The excessive growth, or "blooms", of algae promoted by these phosphates change the water quality in lakes and ponds which can kill fish.

Evapotranspiration

The process by which the Earth's surface or soil loses moisture by evaporation of water and by uptake and then transpiration from plants.

Event (rainfall)

Single occurrence of a rainfall period before and after which there is a sufficient dry period for runoff and discharge from the drainage system to cease.

Everyday events

Events with a return period of less than one year (100% probability). These events typically cause pollution.

Exceedance

An event which has a result which exceeds a set target level, or in the case of drainage networks, a flow which exceeds the capacity of the sewers, causing surcharging and/or flooding.

Exceedance flow

Excess flow that appears on the surface once the conveyance capacity of the minor system is exceeded.

Exceedance flow route

Design and consideration of above-ground areas that act as pathways permitting water to run safely over land to minimise the adverse effect of flooding on people and property. This is required when the design capacity of the drainage system (SuDS or traditional drainage) has been exceeded.

Extended Detention basin

A detention basin where the runoff is stored beyond the time for attenuation. This provides extra time for natural processes to remove some of the pollutants in the water.

Extreme events

Rainfall events of great than 30 year return period (3.3% probability). Can often lead to major flooding.

FEH

Flood estimation handbook, produced by Centre for Ecology and Hydrology, Wallingford (formerly the Institute of Hydrology)

Filter drain

A linear drain consisting of a trench filled with a permeable material, often with a perforated pipe in the base of the trench to assist drainage.

Filter strip

A vegetated area of gently sloping ground designed to drain water evenly off impermeable areas and to filter out silt and other particulates.

Filtration

The act of removing sediment or other particles from a fluid by passing it through a filter.

Fines

Small soil particles less than 63 micron in size.

First flush

The initial runoff from a site or catchment following the start of a rainfall event. As runoff travels over a catchment it will pick up or dissolve pollutants and the "first flush" portion of the flow may be the most contaminated as a result. This is especially the case in small or more uniform catchments; however, in larger or more complex catchments pollution wash-off may contaminate runoff throughout a rainfall event.

Flood and Water Management Act

Legislation to clarify the framework for managing flood risk (particularly surface water management) in England and Wales (Defra, 2010).

Flood frequency

The probability of a flowrate being exceeded in any year.

Flood routeing

Design and consideration of above-ground areas that act as pathways permitting water to run safely over land to minimise the adverse effect of flooding. This is required when the design capacity of the drainage system has been exceeded.

Floodplain

Land adjacent to a watercourse that would be subject to repeated flooding under natural conditions (see Environment Agency’s Policy and practice for the protection of flood plains for a fuller definition).

Flow control device

A device used for the control of surface water from an attenuation facility, eg a weir.

Forebay

A small basin or pond upstream of the main drainage component with the function of trapping sediment.

Foul drainage

The infrastructure that drains the water and sewage that is discharged from within houses.

Freeboard

Distance between the design water level and the top of a structure, provided as a precautionary safety measure against early system failure.

Geocellular structure

A plastic box structure used in the ground, often to attenuate runoff.

Geogrid

Plastic grid structure used to increase strength of soils or aggregates.

Geomembrane

An impermeable plastic sheet, typically manufactured from polypropylene, high density polyethylene or other geosynthetic material.

Geotextile

A plastic fabric that is permeable.

Green infrastructure

A strategically planned and delivered network of natural and manmade green and blue spaces that sustain natural processes. It is design and managed as a multifunctional resource capable of delivering a wide range of environmental and quality of life benefits for society.

Green roof

A roof with plants growing on its surface, which contributes to local biodiversity. The vegetated surface provides a degree of retention, attenuation and treatment of rainwater, and promotes evapotranspiration. Sometimes referred to as an alternative roof.

Greenfield runoff

The runoff that would occur from the site it its undeveloped and undisturbed state. Greenfield runoff characteristics are described by peak flow and volumes of runoff for rainfall events of specified duration and return period (frequency of occurrence).

Greenfield runoff volume

Volume of runoff from the site in its state before any previous development, for the 1 in 100 year rainfall event.

Greywater

Waste water from baths, showers, sinks (kitchen sinks are excluded due to high organic content), and domestic appliances before it reaches the sewer.

Groundwater

Water that is below the surface of ground in the saturation zone.

Groundwater protection zone

Areas that influence water supply boreholes where groundwater must be protected from pollution. These are defined by reference to travel times of pollutants within the groundwater.

HOST

(Hydrology of Soil Types) A classification used to indicate the permeability of the soil and the percentage runoff from a particular area.

Heavy metal

Loosely, metals with a high atomic mass (sometimes given as metals with an atomic mass greater than that of calcium), often used in discussion of metal toxicity. No definitive list of heavy metals exists, but they generally include cadmium, zinc, mercury, chromium, lead, nickel, thallium, and silver. Some metalloids, eg arsenic and antimony, are classified as heavy metals for discussion of their toxicity.

Highway authority

A local authority with responsibility for the maintenance and drainage of highways maintainable at public expense.

Highway drain

A conduit draining the highway on a highway maintainable at the public expense it is vested in the highway authority.

Hydraulics

Hydraulics is another term for fluid mechanics used in the context of water engineering, and is the study of flows. In the context of this report, hydraulics covers the storage, conveyance and control of flows within the proposed drainage network.

Hydrodynamic separators

Proprietary systems designed to remove floating debris, sediments and other associated pollutants from surface water, using fluid dynamics to separate the solids from liquids.

Hydrograph

A graph illustrating changes in the rate of flow from a catchment with time.

Hydrology

The study of the waters of the Earth, their occurrence, circulation, and distribution; their chemical and physical properties; and their relation with the environment, including their relation to living things.

Impermeable

Will not allow water (or any liquid) to pass through it.

Impervious

Will not allow water (or any liquid) to pass through it.

Infiltration (to a sewer)

The entry of groundwater to a sewer.

Infiltration (to the ground)

The passage of surface water into the ground.

Infiltration basin

A dry basin designed to store and promote infiltration of surface water to the ground.

Infiltration device

A device specifically designed to aid infiltration of surface water into the ground.

Infiltration trench

A trench, usually filled with permeable granular material, designed to promote infiltration of surface water to the ground.

Interception storage

The capture and infiltration of small rainfall events up to about 5mm. Normally prevents surface water from the runoff reaching the drainage system.

Interflow

Shallow infiltration to the soil, from where it may infiltrate vertically to an aquifer, move horizontally to a watercourse or be stored and subsequently evaporated.

Land use

Catchments or development areas zoned based on economic, geographic or demographic use of land, such as residential, industrial, farm, commercial.

Lateral drain

(a) That part of a drain which runs from the curtilage of a building (or buildings or yards within the same curtilage) to the sewer with which the drain communicates or is to communicate; or

(b) (if different and the context so requires) the part of a drain identified in a declaration of vesting made under section 102 or in an agreement made under section 104 of the Water Industry Act 1991.

Level of protection

A target level in which no flooding should occur, eg a 0.02 probability of a property being flooded.

Level of service

A level of water, usually specified as a return period to which an urban drainage system is designed to provide as a minimum, referred to as a target level.

Linear pond (or wetland)

An alternative to swales in managing runoff from roadways and hard standings. They are vegetated open channels that provide storage, conveyance and some treatment.

Local development framework (LDF)

A non-statutory term used to describe a folder of documents that includes all the local planning authority’s local development documents (LDDs). The LDF will comprise the statement of community involvement, the local development scheme and the annual monitoring report.

Long term storage

Provided to allow volumetric runoff control during extreme rainfall by discharging water very slowly during and after the storm event.

Low Impact Development (LID)

An approach to land development (or re-development) that works with nature to manage stormwater as close to its source as possible.

Major system

The system of above ground flood pathways, including both open and culverted watercourses.

Management train

The management of surface water runoff in stages as it drains from a site (see SuDS Management Train).

Master plan

A master plan includes both the process by which organisations undertake analysis and prepare strategies, and the proposals that are needed to plan for major change in a defined physical area.

Mean Annual Flood

Referred to as QBAR, or the mean annual flood, is the value of the average annual flood event recorded in a river. This flow rate is used to provide a measure of the greenfield runoff performance of a site in its natural state to enable flow rate criteria to be set for post development surface water discharges for various return periods.

Micropool

Pool at the outlet to a pond or wetland that is permanently wet and improves the pollutant removal of the system.

Minor system

The formal or designed drainage system.

Multi-functional, public open space.

Any space outside the curtilage of an individual property unit may be suitable for SuDS. For example, car parking and bike paths can be surfaced with permeable paving and may have a drainage channel, a play space may provide storage for extreme rainfall.

National Standards for Sustainable Drainage

Sometimes referred to as the National Standards. A regulatory document providing standards and guidance on the design, construction and maintenance of SuDS for approval and adoption by the SuDS approval body (in accordance with Schedule 3 of the Flood and Water Management Act, Defra 2010). (Only available in draft form)

Nibbling

Also referred to as opportunistic retrofitting. Where the opportunity to retrofit surface water management measures arises on the back of other drivers, such as regeneration or small scale improvements. These may occur within a neighbourhood, or locally on a plot level.

Off-line

A conveyance or storage component which bypasses the main conveyance route for the drainage system.

On-line

A conveyance or storage component which forms part of the main conveyance route for the drainage system.

Organic pollution

A general term describing the type of pollution that, through the action of bacteria, consumes the dissolved oxygen in rivers. The effects of organic pollution are described by the levels of bio-chemical oxygen demand, ammonia, and dissolved oxygen found in a waterbody.

Orifice plate

Structure with a fixed aperture to control the flow of water.

Outfall

The flow of excess water from a structure when its capacity is exceeded.

Overland flow

The flow of water over the surface from rainfall runoff before it enters the drainage system, or from excess flow passing out of a drainage component that is full.

Pathogen

An organism that causes disease.

Pathway

The route by which potential contaminants may reach targets.

Peak flow

The maximum volume of water flowing in a watercourse or sewer over a certain period of time following a rainfall event.

Peak greenfield runoff rate

Peak rate of runoff from the development site in its naturally vegetated state before any previous development.

Peak previously-developed runoff rate

Peak rate of runoff from the development site on previously developed land in its previously-developed state taking into account any known surface water drainage system.

Penstock

A sliding plate which moves vertically to vary the size of an aperture (or close it completely).

Percentage runoff

The proportion of rainfall that runs off a surface.

Percolation

The passing of water (or other liquid) through a porous substance or small holes (eg soil or geotextile fabric).

Permeability

A measure of the ease with which a fluid can flow through a porous medium. It depends on the physical properties of the medium, for example grain size, porosity, and pore shape.

Permeable pavement

A permeable surface that is paved and drains through voids between solid parts of the pavement.

Permeable surface

A surface that is formed of material that is itself impervious to water but, by virtue of voids formed through the surface, allows infiltration of water to the sub-base through the pattern of voids, for example concrete block paving.

Pervious surface

A surface that allows inflow of rainwater into the underlying construction or soil.

Piped system

Conduits generally located below ground to conduct water to a suitable location for treatment and/or disposal.

Pond

Permanently wet depression designed to retain stormwater and permit settlement of suspended solids and biological removal of pollutants.

Porosity

The percentage of the bulk volume of a rock or soil that is occupied by voids, whether isolated or connected.

Porous asphalt

An asphalt material used to make pavement layers pervious, with open voids to allow water to pass through (previously known as pervious macadam).

Porous paving

A permeable surface that drains through voids that are integral to the pavement.

Porous surface

A surface that infiltrates water to the sub-base across the entire surface of the material forming the surface, for example grass and gravel surfaces, porous concrete and porous asphalt.

Potable/mains water

Water company/utility/authority drinking water supply.

Practicality

To determine what is reasonably practicable on a particular site involves gathering data and working through a structured series of decisions.

Prevention

Site design and management to stop or reduce the occurrence of pollution of impermeable surfaces and to reduce the volume of runoff by reducing impermeable areas.

Previously developed land

Land which is or was occupied by a permanent structure, including the curtilage of the developed land and any associated fixed surface infrastructure. The definition of developed land includes defence buildings, but excludes:

Land that is or has been occupied by agricultural buildings.

Land that has been developed for minerals extraction or waste disposal by landfill purposes where provision for restoration has been made through development management procedures.

Land in built-up areas such as private residential gardens, parks, recreation grounds and allotments, which although it may feature paths, pavilions and other buildings, has not been previously developed.

Land that was previously-developed but where the remains of the permanent structure or fixed surface structure have blended into the landscape in the process of time (to the extent that it can reasonably be considered as part of the natural surroundings).

Where development takes place on land which was previously partially developed, a pro-rata approach should be taken.

Previously-developed runoff volume

Volume of runoff from the site in its previously developed state, for 1 in 100 year rainfall event.

Probability

The estimated likelihood of a storm event e.g. a 1 in 100 year flood event is one that is expected to be equalled or exceeded once every 100 years; it also has a 1% chance of occurring in any one year.

Proper outfall

An outfall to a watercourse, public sewer and in some instances an adopted highway drain. Under current legislation and case law, the existence of a proper outfall is a prerequisite in defining a sewer.

Rain garden

A planted basin design to collect and clean runoff (normally from a roof, or hardstanding with low risk of pollution).

Rainfall event

A single occurrence of rainfall before and after which there is a dry period that is sufficient to allow its effect on the drainage system to be defined.

Rainwater harvesting or rainwater use system

A system that collects rainwater from where it falls rather than allowing it to drain away. It includes water that is collected within the boundaries of a property, from roofs and surrounding surfaces.

Receiving waters

Water body (river, lake or watercourse) which receives a discharge from point or non-point sources.

Recharge

The addition of water to the groundwater system by natural or artificial processes.

Recurrence interval

The average time between runoff events that have a certain flow rate, e.g. a flow of 2 m/s might have a recurrence interval of two years in a particular catchment.

Reed bed

Area of grass-like marsh plants, primarily adjacent to freshwater. Artificially constructed reed beds can be used to accumulate suspended particles and associated heavy metals, or to treat small quantities of partially treated sewage effluent.

Regional control

Management of runoff from a site or several sites, typically in a balancing pond or wetland.

Retention pond

A pond where runoff is detained for a sufficient time to allow settlement and biological treatment of some pollutants.

Retention time

The length of time that runoff is stored or detained to allow for settlement, or possibly biological action, to occur.

Return period

Refers to how often an event occurs. A 100-year storm refers to the storm that occurs on average once every hundred years. In other words, its annual probability of exceedance is 1 per cent (1/100). A 500-year storm is the storm expected to occur once every 500 years, or has an annual probability of exceedance equal to 0.2 per cent (1/500).

Rill

Open surface water channels with hard edges (they can be planted to provide a cleaning function).

Riparian

Of, on, or relating to the banks of a natural course of water.

River basin

The area of land from which all surface run-off flows through a sequence of streams, rivers and, possibly, lakes into the sea at a single river mouth, estuary or delta.

Runoff

Water flow (including flow from snow and other precipitation) over the ground surface which has not entered the drainage system. This occurs if the ground is impermeable, is saturated or rainfall is particularly intense. (Sometimes referred to as surface water runoff, surface runoff).

Runoff coefficient

A measure of the amount of rainfall that is converted to runoff.

SOIL

Soil Index Value obtained from the WRAP soil classification, used in the Wallingford Procedure to calculate the treatment volume.

Sediments

Sediments are the layers of particles that cover the bottom of waterbodies such as lakes, ponds, rivers and reservoirs.

Separate sewer

A sewer for surface water or foul sewage, but not a combination of both.

Sewer

A pipe or channel taking domestic foul and/or surface water from buildings and associated paths and hard-standings from two or more curtilages and having a proper outfall.

Sewerage undertaker

This is a collective term relating to the statutory undertaking of water companies that are responsible for sewerage and sewage disposal including surface water from roofs and yards of premises.

Sewers for Adoption

A guide agreed between sewerage undertakers and developers (through the Home Builders Federation) specifying the standards to which private sewers need to be constructed to facilitate adoption.

Sewers for Scotland

The objective is the same as Sewers for Adoption (ie defining construction standards for drainage systems), but varying in technical legal detail for Scotland and providing requirements for SuDS.

Silt

The generic term for waterborne particles with a grain size of 4-63 mm, ie between clay and sand.

Site control

Management of water in a local area or site, eg routing water from building roofs and car parks to a large soakaway, infiltration or detention basin.

Soakaway

A sub-surface structure into which surface water is conveyed, designed to promote infiltration.

Source control

The control of runoff at or near its source.

Source –Pathway – Receptor

An approach for establishing, assessing and communicating risk relationships.

Storm events

Events occurring between 1 in 1 year (100% probability) and 1 in 30 year return period (3.3% probability). These events are typically what urban drainage systems (below ground) are designed up to, and which flooding occurs.

Where a driver for better surface water management is present and a wider approach across a neighbourhood or catchment is taken to retrofit surface water management measures.

SuDS approval body (SAB)

An organisation formed by an upper tier or unitary authority responsible for the approval and adoption of drainage schemes in accordance with the National Standards for Sustainable Drainage (once introduced).

SuDS management train

The management of runoff in stages as it drains from a site. A range of SuDS components are used to maximise the hydraulic and water quality management benefits.

Sub-base

A layer of material on the sub-grade that provides a foundation for a pavement surface.

Sub-catchment

A division of a catchment, to allow runoff to be managed as near to the source as is reasonable.

Sub-grade

Material, usually natural insitu, but may include Capping layer, below Formation level of a Pavement.

Substrate

An underlying layer; a substratum

Sump

A pit that may be lined or unlined and is used to collect water and sediments before being pumped out.

Surface flood pathways

Routes in which exceedance flows are conveyed on the ground.

Surface water

Water from precipitation which has not seeped into the ground and which is discharged to the drainage system.

Surface water management

The management of runoff as it drains from a site or catchment.

Suspended solids

General term describing suspended material. Used as a water quality indicator.

Sustainable drainage systems (SuDS)

A sequence of management practices and control structures designed to drain systems (SuDS) surface water in a more sustainable fashion than some conventional techniques. Sometimes referred to as SuDS or Sustainable Urban Drainage.

Swale

A shallow vegetated channel designed to conduct and retain water, but may also permit infiltration. The vegetation filters particulate matter.

Target level

A defined level of protection that is satisfied when failure occurs less frequently than the specified level.

Temporary storage

Storage of the excess water for high intensity events, where the drainage system is temporarily overloaded.

Treatment

Improving the quality of water by physical, chemical and/or biological means.

Treatment stage

A sustainable drainage component that protects or improves surface runoff by reducing suspended sediments or contaminants.

Treatment train

A series of SUDS components, each designated to treat a different aspect of runoff that are implemented together to maximise their effectiveness. (See SuDS management train)

Treatment volume

The volume of water provided in a pond to provide dilution and detention to inflowing surface runoff to provide partial treatment.

Trigger level

When failure occurs more frequently than the identified level of protection.

Turbidity

Reduced transparency of a liquid caused by the presence of un-dissolved matter.

Underground storage

Underground structure, often constructed using concrete, grp, steel tanks or plastic void formers. They provide hydraulic attenuation, but do not treat the runoff.

Vortex flow control

The induction of a spiral/vortex flow of water in a chamber used to control or restrict the flow.

WRAP

(Winter Rain Acceptance Potential) Classification used to calculate the permeability of soils and the percentage runoff from a particular area.

Water Framework Directive (WFD)

A Directive designed to integrate the way waterbodies are managed across Europe. It requires all inland and coastal waters to reach “good status” by 2015 through a catchment-based system of river basin management plans, incorporating a programme of measures to improve the status of all natural water bodies.

Water Quality Treatment volume

The permanent volume of water in a pond between rainfall events which is designed to provide dilution and adequate detention of surface water runoff to allow sedimentation along with other treatment processes to occur to provide partial treatment of the runoff before it is discharged from the site.

Water Sensitive Urban Design

A process that integrates the different water streams (surface water, waste water and water supply) within the water cycle and places water management at the heart of urban design (and vice-versa). SuDS make a contribution to WSUD, but they are not WSUD in its entirety.

Water quality

The chemical and biological content of water, usually compared to defined standards, set by legislation and enforced by environmental regulators.

Water table

The point where the surface of groundwater can be detected. The water table may change with the seasons and the annual rainfall.

Watercourse

A term including all rivers, streams, ditches, drains, cuts, culverts, dykes, sluices, and passages through which water flows.

Wetland

Flooded area in which the water is shallow enough to enable the growth of bottom-rooted plants.

Whole life cost (WLC)

The present day value of total costs of a structure throughout its likely operating life.